The invention relates to an illumination system comprising, in this order, a radiation source and an integrator system.
The invention also relates to an image projection device provided with such an illumination system.
An image projection device with an illumination system of the type described in the opening paragraph is known from, inter alia, U.S. Pat. No. 5,098,184. The image projection device described in this Patent comprises an illumination system for supplying an illumination beam and an image display system having at least one display panel for modulating this light beam in conformity with image information to be projected. In addition to a radiation source, a reflector behind the radiation source and a condensor lens, the illumination system also includes an integrator system. A further lens ensuring that all re-images are superimposed in the plane of the display panel is present behind the integrator system.
If the display panel adds image information to the light beam by means of polarization modulation, the liquid crystalline display panel is generally situated between two polarizers. If unpolarized light is incident on the display panel, substantially half of it will be absorbed by the first polarizer and will thus be lost for the formation of the image. Moreover, the absorption gives rise to heating of the polarizer and the display panel, so that the display panel may be damaged.
It is an object of the invention to provide an illumination system in which substantially the complete unpolarized light beam emitted by the radiation source is converted into a light beam having one and the same direction of polarization, without any noticeable loss of light occurring. In this way, an illumination system having a relatively large light output is realized without extra efforts to be made for the radiation source itself.
To this end, the illumination system according to the invention is characterized in that first reflecting means are present in the exit plane of the integrator system, in that the illumination system further includes second reflecting means in the form of a reflective polarizer, in that an optical system for at least partly imaging the exit surface of the integrator system via the second reflecting means on the first reflecting means is present between the first reflecting means and the second reflecting means, and in that polarization-converting means are present between the first and the second reflecting means.
The present invention is based on the recognition that a light-obstructing or light-absorbing element having the function of eliminating one of the directions of polarization from a light beam is replaced by a reflecting element, so that the unwanted direction of polarization can be recuperated, converted by the polarization-converting means into the desired direction of polarization and thus be given another chance to contribute as yet to the light output.
The above-mentioned inventive idea is realized by a combination of first and second reflecting means, between which the unwanted light is reflected, and polarization-converting means for at least partly converting unwanted light into wanted light. The unwanted light is thus recuperated instead of being absorbed or obstructed, so that a relatively high light output is realized without further complicating the illumination system.
The optical system ensures that the light coming from the second reflecting means at least partly reaches the first reflecting means, and vice versa.
The integrator system may be implemented in different manners.
A first embodiment of the illumination system according to the invention is characterized in that the integrator system comprises a first and a second lens plate, in which the lenses of the second lens plate jointly constitute the exit surface of the integrator system.
In this case, the plane of the second lens plate is situated in the exit plane of the integrator system. The exit surface of the integrator system is situated in the exit plane. The second lens plate surface contributing to the integrator function, in other words, the surface occupied by the lenses of the second lens plate, is understood to be the exit surface.
Each lens plate is provided with a matrix of lenses, with each lens of the first lens plate imaging the radiation source on a lens of the second lens plate. Each lens of the second lens plate images the corresponding first lens on the record carrier which is illuminated by the illumination system.
A further embodiment of the illumination system according to the invention is characterized in that the first reflecting means have reflecting elements which are present on the second lens plate.